busybox: handle syslog

busybox: handle syslog

If CONFIG_KLOGD is not enabled, then the related service file should
not be installed, The error message is below:

    Cannot add dependency job for unit busybox-klogd.service,
    ignoring: Unit busybox-klogd.service failed to load:
    No such file or directory.

So we should first check the configuration before we install these
service files.

Signed-off-by: Yadi.hu <yadi.hu@windriver.com>
Signed-off-by: Mingli Yu <mingli.yu@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>